This invention relates generally to a raster output scanner of a laser printing device and more particularly concerns a method in which pseudo optical retro reflectors are used for monitoring a scanning laser spot position in both horizontal and vertical axes in real time.
A conventional raster output scanner utilizes a light source, pre-scan optics, a modulator, a multi-faceted rotating polygon mirror as the scanning element, and post scan optics. The light source, which can be a laser source, produces a light beam and sends it to the modulator. The modulator receives electronic pixel information for modulating the light beam. The modulated light beam will be directed onto the rotating polygon. At the strike of the modulated light beam, the rotating polygon reflects the modulated light beam and causes the reflected light beam to revolve about an axis near the center of rotation of the rotating polygon and scan a straight line. This reflected light beam can be utilized to scan a document at the input of an imaging system or can be used to impinge upon a photographic film or a photosensitive medium, such as a xerographic drum at the output of the imaging system.
Laser printers using rotating polygons often include a line start system to accurately position the start of scan of each line in the raster plane. A line start system is necessary to avoid facet-to-facet variations that can cause unevenness in the scan lines. Many line start systems monitor the position of the scanned beam for starting the printing of the line in the raster plane. However, such systems can require multiple beams and synchronization signals to compensate for facet defects which affect the line start signal. U.S. Pat. No. 4,980,549 discloses such a system. However, such systems are complicated and only monitor the position of the beam in the horizontal plane, and then only at start of scan and/or end of scan.
Accordingly, it is the primary aim of the invention to provide a simplified method of detecting the position of a laser spot in both the horizontal and vertical planes.
Further advantages of the invention will become apparent as the following description proceeds.